This is the 16650th most frequent Russian word.
серьезная
Serious.
Here, 'серьёзная' is used as an adjective describing the book, indicating its profound or thought-provoking nature.
Это серьёзная книга, требующая внимательного изучения.
This is a serious book that requires careful study.
In this sentence, 'серьёзная' qualifies a problem, emphasizing its importance or difficulty.
У него серьёзная проблема, которую нужно решить как можно скорее.
He has a serious problem that needs to be addressed as soon as possible.
Here, 'серьёзной' describes a person's demeanor or attitude, highlighting a focused or earnest behavior.